Crystal oscillator circuits



- NOV. 2,1948. 1 N AR 2,452,951

CRYSTAL OSCILLATOR CIRCUITS Filed April 2'7, 1943 Fig. l.

Ifiventor:

Donald E. Norgaard,'

9 His Attorngg;

Patented Nov. 2, 1948 CRYSTAL OSCILLATOR CIRCUITS Donald E. Norgaard, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application April 27, 1943, Serial No. 484,733

My invention relates to oscillator circuits and in particular to piezo-electric crystal controlled oscillator circuits. It is well known that a quartz crystal by itself exhibits the electrical properties of both a series resonant, circuit and a parallel resonant circuit and, customarily, is represented electrically as a series circuit of inductance, capacitance and resistance, this series circuit being shunted by a capacitance. When supported in a holder, the combination of the holder and crystal likewise exhibits the electrical properties of both series and parallel resonance, the holder appearing as additional capacitance in shunt to the above-mentioned circuit. It is an object of my invention to provide an improved crystal con-trolled oscillator circuit in which the crystal operates at its series resonant point.

Another object of my invention is to provide an improved oscillator circuit which is substantially free from any shift in frequency as the tuning is adjusted.

A further object of my invention is to provide an improved crystal controlled oscillator circuit in which the frequency of oscillations produced is substantially unaffected by either the capacity of the crystal holder or the intensity of theopcrating potential applied to the associated vacuum tube circuit.

One of the features of my invention is the employment of a piezo-electric crystal, operating at its series resonant point and connected between the input and output circuits of an electron discharge device in an oscillating circuit, as feedback means to sustain oscillations of the circuit as well as to control the frequency of such oscillations.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 represents a crystal oscillator circuit constructed according to my invention and Figs. 2 and 3 are modifications of the circuit of Fig. 1.

Referring to Fig. l, I have shown a crystal oscillator circuit comprising an electron discharge device I!) having an" anode l l, a'cathode l2, and a control electrode l3. A tuned input circuit comprising a fixed inductance M and a variable capacitance i5 is connected between control electrode l3 and ground through a grid leak resist- 18 Claims. (Cl. 250-36) ance 16 having a condenser I! in shunt therewith. Cathode IZ of device is connected to ground through a cathode biasing resistance 18. Anode l I is supplied with operating potential from any suitable source, such as the battery through a decoupling resistance 2|, and a capacitance 22 is connected between the anode I l of the oscillator circuit and ground to by-pass alternating currents from the anode circuit of device 10.

As means to sustain oscillations in the oscilpiezo-electric crystal exhibits the electrical properties of a series resonant circuit, as well as a parallel resonant circuit, the series resonant point being substantially unaffected by adjustment of the crystal holder, spacing or the, capacitance of the crystal holder and associated'circults, whereas the parallel resonance point of the crystal is a function of these parameters. In the circuit of Fig. 1, since the piezo-electric crystal 25 exhibits asharp series resonant characteristic, the amount of feedback from the output circuit of the device ID to the input circuit is a function of frequency. This feedback is greatest when the impedance of the crystal is lowest, that is, when the crystal is operated at its series resonant frequency. Since the inductance I4 and capacitance l5 are tuned approximately to the samefrequency as the crystal, the loading effect of this tuned circuit on the device ID is small at this operating frequency. Also, since the feedback path from the cathode to the control electrode through crystal 25 is much more sharply tuned than the tuned circuit composed of elements l4 and I5, the crystal 25 has a controlling effect on the frequency of oscillations generated in the oscillating circuit.

In order to sustain oscillations in the circuit thus described the loop gain through the feedbacklatory circuit is automaticallymaintained at an average' value I just sufiicient to sustain stable oscillations of a'certain desired strength by means of the grid leak resistance l6 and the condenser combination li.

Also, when the current in grid leak resistor I6 is zero at the initiation of operation enough loop gain is provided in the oscillatory circuit to permit oscillation to start. To this end, the oathode biasing-resistancel8 is ofisllcha value as to allowoscillations to build up to a substantially fixed value before regulation of gain by the grid leak resistance i6 is started, the fixed value being such that a loop gain of greatcrthan unity is attained when elements It and i resonate at substantially the frequencyof the crystal.

In the modification of the oscillatory circuit shown in Fig. 2, an electron discharge device 30 of the pentode type is employed. The connections of the input circuit between the control electrode 3| and the cathode 32 are the same as areshown. in the circuit of Fig. 1 and corresponding parts ottheitwo circuits have been designated-by corresponding reference numerals. The screen electrode .3-3 of the device .39 functions-as ananode for-the oscillatory circuit. .It is supplied with operating potential from the battery through a decoupling and voltage dropping resistance and is by-passed to ground-for alternating currents by means or a capacitance 35. The anode fifixmay be supplied with operating-potential from the battery-.flfl throug-h a load circuit 31. The suppressor grid :38 is connected directlyto ground, instead of to the cathode, to prevent coupling between the cathode and anode 36 due .to the capacity between the suppressor grid, 38 and anode36 in the separation of the oscillatory circuit of Fig. 2, the-outputcurrents flowing between .the anode 136 and cathode .32 vary in frequency in accordance with the oscillations which are developed between the-screenelectrode 33, operating as an anodend tho e A i the op ation of the circuitcfFig. 1, theJooDgain of theoscillatory circuit of Fig. 2. is automatically maintained at an avera e-valued unity, that is, ayalue just su i' cic tt su i abl -cs llaticnahy mean -cf the ri eak r sistan e vI 6 an l apacitance .1. The cathodebiasing resistance! Bis. of such value hat the b a p ovided by the cathod curren flowing therethrough allows the oscillations. to build lip tOa substantially fixed value before the eridlcak r i nce it funct ns to re u ate t gain of this portion of the circuit. The tuned circuit [4, I5 is adjusted to resonate substantially at'the series resonant frequency of the crystal 25 so that the loop ain of the oscillating circuit,'the value of whichis a functionof frequency, may be greatest at the-series resonant frequency of "the crystal and of such value at this "frequency that oscillations are sustained.

"In Fig. 3, I'haveshownanother modification of my oscillator circuit in which the tuned input circuit connected'between the control "electrode 13 of electron discharge device II] and ground comprises an inductance 40 having in shunt therewith the serially connected capacitances M and 42. The inductance 40 may be variable in order to adjust the tuningof the circuit, or-all-of =t e reactance elements may have preselected values such that the-tuned circuit resonates substantially at the series resonant frequency of; the crystal 25. The piezos-eleotric crystal -25 'isconnectedzbetween the cathode I2 of thez-devicez I'll and "the common point of capacitances A l ,and 42 to dunction'a-sra feedback circuit betweentheginput andmutnutcircuitsiof .devl ce lifl. .-S;ince::the tuned input circuit operates at the series resonance frequency of crystal 25, this feedback circuit exhibits a sharp resonant characteristic. In the oscillatory circuit, the values of capacitances 42 and ti are adjusted so that the loop gain of the oscillating circuit may be of such value at the'series resonant frequency of the crystal that oscillations are s-ustainede In all other-respects this circuit functions essentially thesame as the circuit of Fi 1 "It is seen that the crystal oscillator circuit of myinventlon, b operating at the series resonant point of the crystal, provides oscillations which are-substantially free from frequency shift during tuning-adjustments. The frequency of oscillations produced is sustantially unaffected by either the capacityof the crystal holder 26 or the intensity of the operating potential applied by meansof source 20. The resultant oscillatory circuit, therefore, is both simple in construction and, because the feedback path between the input and output circuits is tuned sharply by the crystal to the series resonant frequency of the crystal, is stable in operation.

While I have shown a particular embodiment of my invention, it will of course be understood that I'do not-wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of 'theUnited-States is:

1. An oscillatorycircuit comprising an electron discharge device having an anode, a cathode, and control electrode, a-tuned circuit connected to said control electrode, an impedance connected between said tuned circuit and said cathode. and feedback means comprising a piezo-electric crystal connected between said cathode and a fixed point on said tuned circuit, said tuned circuit being tuned substantially to the series resonance frequency of said. crystal.

2. linoscilla cry ci cu comprising ancl ctr n d' chargcxde cchavine;an an d a cath d and a control .-electro d.e, .a tuned circuit connected betweensaid.controlelectrode and a point of fixed potential, an impedance connected between said cathode ,a-ndsaid pointg-and feedback means comprising a -piezo-electric crystal connected between said cathode and afixed-point on said tuned circuit, said tuned-circuitbeing tuned substantially tot-heresonance frequency of said crystal.

3; Anpscillatory circuit comprising an electron discharge device having an anode, a cathode, and a control electrode, a tuned circuit connected between saidcontrol electrode and a Point of fixed potential, animpedance connected between said cathode and said point, and feedback means comprisinga piezo-eleCtric crystal connected between said cathode anda fixed point on said tuned circuit, said tuned circuit being tuned substantially to the series resonance frequency of said crystal.

4.. An oscillatory circuit comprising an electron means-comprising a piezo-electriccrystal con-- nee-ted between-said cathodeand a fixed point on said: inductance, said inductance and said capacitance-hayingsuch values as to resonate substantially ;at the series resonance frequency:

of said crystal.

encased An osci-llatorywcircuit comprising-an electrons discharge. device .having;aanv anode, a cathode, and .a control electrode; an: inductance having .a capacitancea in." shunt therewith, connected betweensaidacontrol electrodeand a-point ofyfixed potential, an impedanceconnected sbetween said cathode I and said :point, a eby pass. capacitance connected. betweenlsaid v anode and said point,

til

and feedback means comprising,as-piezoeelectrie crystalaconnected betweensaid cathode and a frequency of;said crystal.

6;: In: tin-oscillator, the combination; of an elec--. tron discharge devicerhavingan:anode,1a cathode,

and a control Lelectmde, an input circuit; connected to Lsaid electrode, said circuit comprising i a iseriallypconnected resistance and inductances a second resistance connectedbetweensaid-rim ductance and said cathode, feedback means com prisinga piezo-electriccrystal connected between said cathode and a tap on said input circuit, and means for resonating saidinductance substantiallyat the series resonance frequency of .said crystal.

7. In an oscillator, the combination of an electron dischargedevice having an anode, a cathode,

and aicontrol electrode, a-bypass capacitance connected between said anode and ground-ta'resistancei connected between; .said, cathode: and ground-aninput circuit connectedcbetween saidcont-rol electrode and ground,.-,said ,-input circuit comprising a serially connected resistance and inductance, feedback means comprising a piezoelectric crystal connected between said cathode and a tap on said input circuit, and means for resonating said inductance substantially at the resonance frequency of said crystal.

8. In an oscillator, the combination of an electron discharge device having an anode, a cathode, and a control electrode, a by-pass capacitance connected between said anode and ground, a resistance connected between said cathode and ground, an. input circuit connected between said control electrode and ground, said input circuit comprising a serially connected resistance and a pair of capacitances. feedback means comprising a piezo-electric crystal connected between said cathode and a point between said capacitances, and means for resonating said capacitances substantially at the resonance frequency of said crystal.

9. A piezo-el'ectric crystal oscillation generator comprising an electron discharge device having an anode, a cathode, and a control electrode, an input circuit connected between said control electrode and cathode, a by-pass capacitance connected between said anode and said cathode, said input circuit including a tuned circuit, and feedback means connected between said cathode and said input cricuit, said means comprising a piezo-electric crystal, said tuned circuit being adjusted to resonate at substantially the series resonance frequency of said crystal, and said ing. a-,seria'lly.uconnectedresistance. and inducts ance; feedback means. comprising a piezo-electric crystal: connected between said cathode and a tap on said-.inductance, and a capacitance con-- ,nected. across said inductance, said capacitance being of such value as to resonate said inductancetsubstantially at the resonance frequency of-said crystal;

ll; Inan oscillator circuit, the combination of an ;:electron* discharge device having an anode,

a cathode, and a control electrode; a circuit connected between said anode and ground, a resistance connected between said cathode and ground, anxinput circuit connected between said control .electrodeancl ground, said input circuit c0mpris-' ing a pair of capacitances having a resistance in series therewith, feedback means comprising a piezo-electric crystal connected between said cathodeand the common point of said capacitances; and an inductance connected across said capacitances, said inductance being of such value as to resonate with said capacitances substantiallyat the resonance frequency of said crystal.

12: Inch oscillator circuit, the combination of an :electron discharge device having an anode, a cathode, a control electrode, and a screen electrode, a-tuned circuit connected between said control electrode and a point of fixed potential, an output circuit connected between said anode and saidxpoint, aresistance connected between said cathode and said point, a by-pass capacitance connected between said screen electrode and :said point, and feedback means comprisinga piezo-electric crystal connected between said cathode and a point on said tuned circuit, said tuned circuit being tuned substantially to the series resonance frequency of said crystal, and said point on said tuned circuit being so selected that the current flowing through said feedback means is sufficient to sustain oscillations in said tuned circuit at said frequency.

13. In an oscillator circuit, the combination of an electron discharge device having an anode, a cathode, a control electrode, and a screen electrode, an inductance connected between said control electrode and apoint of fixed potential, an output circuit connected between said anode and said point, a resistance connected between said cathode and said point, a by-pass capacitance connected between said screen electrode and said point, and feedback means comprising a piezoelectric crystal connected between said cathode and a point on said inductance, and a capacitance connected across said inductance to form therewith a tuned circuit resonating substantially at the series resonance frequency of said crystal, said point on said inductance being so selected that the current flowing through said feedback means is sufiicient to sustain oscillations in said tuned circuit at said frequency.

14. An oscillator circuit comprising an electron discharge device having an anode, a cathode, a control electrode, a screen electrode, and

a suppressor electrode, said suppressor electrode being connected to ground and said screen electrode being connected to ground through a bypass capacitance, a load circuit connected between said anode and ground, a resistance connected between said cathode and ground, a serially connected resistance and inductance connected between said control electrode and ground, feed-back means comprising a piezo-electric crystal connected between said cathode and a tap on said inductance, and means to resonate said inductance substantially at the series resonant frequency of said crystal, said tap being so positioned that the current supplied to said inductance by said feedback means is suflicient to sustain oscillations therein at said frequency.

15. A stabilized crystal-controlled oscillator comprising electron discharge means including a cathode, a control electrode and an anode, means for supplying electrical heating energy to said cathode, means for applying a relatively high potential to said anode, a parallel resonant circuit comprising an inductor shunted by two capacitors in series, a connection from a point on said inductor to said anode, a connection 1' rom another point on said inductor to said control electrode, and a connection including a piezoelectric crystal extending from the mid-junction point of said capacitors to said cathode.

16. A crystal-controlled oscillator comprising electron discharge means including a cathode, a control grid and an anode, means for supplying electrical heating energy to said cathode, means for applying a relatively high potential to said anode, a resonant circuit, means for electrically coupling the grid and anode circuit of said discharge means to produce oscillations in said resonant circuit, and a piezo-electric crystal connected to said cathode and common to said grid and anode circuits.

1'7. A stabilized crystal-controlled oscillator comprising electron discharge means including a cathode, a control electrode and an anode, means for supplying electrical heating energy to said cathode, means for applying a relatively high potential to said anode, a parallel resonant circuit comprising an inductive impedance shunted by a capacitative impedance, a connection from a point on said inductive impedance to said anode,

a connection from another point on said inductive impedance to said control electrode, and a connection including a piezo-electric crystal extending from a mid-point of one of said impedances to said cathode.

18. A stabilized crystal-controlled oscillator comprising electron discharge means including a cathode, a control electrode and an anode, a parallel resonant circuit comprising an inductive impedance shunted by a capacitive impedance, a connection from a point on'one of said impedances to the anode, a connection from another point on the latter impedance to the control electrode, and a connection including a piezo-electric crystal extending from an intermediate point on the latter impedance to the cathode.

DONALD E. NORGAARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,830,329 Nicclson Nov. 3, 1931 2,051,936 Braaten Aug. 25, 1936 2,066,027 Braaten Dec. 29, 1936 2,332,102 Mason Oct. 19, 1943 

